There are a wide variety of applications for nanoporous materials. One application of interest is as dielectrics in the manufacture of integrated circuits. The introduction of porosity into a dielectric generally reduces its dielectric constant, since the dielectric constant of air is close to 1 while that of the common dielectric materials is higher. The reduction of dielectric constant is desirable for a variety of reasons in integrated circuit manufacturing. Another application of porosity is the formation of very small channels in the material which may be used for a variety of purposes. More generally, nanoporous materials have been considered for use in such applications as photonic devices, catalysis, environmental pollution control, separation and isolation of biological molecules, membranes, and energy storage.
The integrated circuit application is a particularly interesting one for nanoporous materials. Integrated circuits consist primarily of transistors and other devices interconnected by wires. The wires are separated from other wires and from the integrated circuit substrate by dielectric films which must be deposited onto the integrated circuit during its manufacturing process. The common dielectric material used in integrated circuits was for decades silicon dioxide, whose dielectric constant k lies between 3.9 and 4.2. Generally speaking the capacitance of wires to ground and to other wires in an integrated circuit will be proportional to the dielectric constant of the dielectric material which separates them. The time for a signal to propagate over a wire in an integrated circuit is related to the product RC, R being the resistance of the wire and C its capacitance to ground. Thus, a reduction of the dielectric constant, leading to a reduction in C, would speed signal propagation and so would tend to make integrated circuits faster. A reduction in dielectric constant would also reduce the power required for signal propagation, which is also approximately proportional to C. Because of this, it is desirable to manufacture integrated circuits which use a dielectric with a significantly lower dielectric constant than silicon dioxide.
A large number of approaches have been attempted to create nanoporous materials. One approach which is generally promising is the use of sacrificial porogens. A sacrificial porogen is a substance which mixes with a polymer matrix out of which the nanoporous material will be made. As the nanohybrid material forms from the matrix, for example through controlled thermal processing, the porogen is dispersed. Once the nanohybrid material is formed, the porogen molecules can then be eliminated, for example by heating, radiation, extraction, or use of a chemical reagent effective to degrade the porogen, leaving voids in their place.
A desirable characteristic for porogens is compatibility with the polymer matrix, allowing dispersal throughout this matrix and thus the creation of porosity spread uniformly throughout the resulting nanoporous material. While the porogens may aggregate somewhat among themselves within the matrix during the formation of the nanoporous material, they preferably do so in a controllable manner and with the formation of nanoscopic domains.
A wide variety of porogens have been proposed and studied. See in this regard W. Volksen et al., “Porous Organosilicates for On-Chip Applications: Dielectric Generational Extendibity by the Introduction of Porosity,” in Low Dielectric Constant Materials for IC Applications, P. S. Ho, J. Leu, W. W. Lee eds., chapter 6 (Springer-Verlag 2002).
Among the classes of porogens which have been studied, those which self-organize have received a fair amount of attention. In particular, as is well known, surfactants in solution may self-assemble into a variety of structures ranging from micelles to bilayers. Surfactants have been successfully employed to create nanoporosity in organosilicate matrices. See, e.g., Volksen et al., supra, at 171-72. Dendrimer self-organization in solution has also been studied. See in this regard J.-W. Weener et al., “Some Unique Properties of Dendrimers Based upon Self-Assembly and Host-Guest Properties,” in J. M. J. Fréchet & D. A. Tomalia eds., Dendrimers and other Dendritic Polymers (Wiley 2001).